The invention relates to a particle accelerator, in particular to an electron accelerator, for creation of a bunched particle beam. Such particle accelerators are employed in particular in medical technology to generate a beam of charged particles. Further fields of application of a particle accelerator of this type include, for example, high-energy physics, in which experimental investigations of material nuclei are carried out, and materials processing by means of ionised radiation.
Particle accelerators accelerate electrically charged particles that are emitted from a particle source, in particular from an electron or proton source, with the aid of electromagnetic fields. Particles with a high kinetic energy, which can be used for a variety of purposes, are obtained by this acceleration.
For medicine in particular, these high-energy-charged particles are of particular interest, since they can be used for radiation therapy. High-energy particles are used in imaging examination methods or for therapy, in particular for cancer therapy, in order to create in turn high-energy electromagnetic radiation. Kinetic energies of 1 MeV or more are required here, with the charged particles being typically accelerated by a series of cavity resonators which work according to the principle of a standing-wave accelerator or of a travelling-wave accelerator, and are grouped into particle packets known as bunches. An electromagnetic wave is set into resonance in the individual cavities of the cavity accelerator, and by exploiting the resonance frequency a high electrical field strength of up to several millions of volts per meter are created with relatively little technical effort, by means of which the electromagnetic particles are accelerated and can be concentrated into particle packets known as bunches. Acceleration energy is transferred to the particles by correctly phased correlation of the field strength of the electromagnetic field oscillating in the cavities and of the electromagnetic particles flying through them. The central components of such a particle accelerator are a particle source and an arrangement of several cavities that are mechanically connected to one another, in which a standing or travelling wave is created in order to accelerate and bunch the particles.
When coupling the electromagnetic wave into the cavities of the resonator structure, the problem arises that a part of the electromagnetic wave is reflected, thus reducing the efficiency of the HF energy supply, which is coupled in for acceleration. In addition, unwelcome higher modes are excited in the resonator cavity, in particular by the particles themselves that are flying through, which prevent an optimum acceleration of following particles. The efficiency of the acceleration mechanism is further reduced by this. Finally, only a small quantity of electromagnetic energy can be supplied into the cavity resonators, so that either a large number of cavities have to be provided, or high power losses occur.
A particle accelerator is described in DE 20 2013 105 829 U1, whose high-frequency energy of an HF source is distributed by a current divider to two HF power couplers, where HF energy is coupled via a first branch into a first section of an accelerator tube by means of a first HF power coupler and a second part of an HF energy is supplied via a reciprocal phase shifter into a second section via a second HF power coupler of an accelerator cavity. The total HF power in the two accelerator tube segments can be controlled by means of the phase shifter. This document therefore addresses the coordination of the phases at both coupling points in order to provide a controllable HF power coupling for particle acceleration. The problem of reflected HE power at the HF power coupler, which results in strain on the HF source, is not discussed.
In addition, a particle accelerator is disclosed by DE 10 2011 076 262, in which electromagnetic energy of an HF source is split via a circulator into two partial energies, a first part being supplied into a first cavity section and a second part being coupled via a phase shifter into a second cavity section of a waveguide structure. Reflected energy from the second or first cavity section can be diverted via a respective HF load. As a result, a separate HF load is required at each coupling point.
A particle accelerator structure is also disclosed by DE 696 34 598 T2, comprising two coupling-in points for HF energy into an accelerator structure. The circuit variant described therein relates to the optimised adjustment of HF powers into two separate accelerator guide sections. Symmetrical hybrids, i.e. directional couplers, are arranged for this purpose, where a synchronisation of the amplitude and phase between the two coupling-in points can be achieved by adjustable and variable short-circuit devices, which can be motor-driven, and by a controller. Two coupling-in points can be operated using highly complex circuitry with scalability for yet more coupling-in points not being offered. Variable short-circuits are provided for coordinating the power of the second accelerator section, necessitating a large number of expensive HF components, and a complex controller is provided.
US 2012/0 326 636 A1 presents a particle accelerator device in which HF power is coupled at one point into an accelerator cavity. An AFC (Automatic Frequency Controller) is provided to regulate the HF power, and serves to control the HF source. The AFC can comprise an adjustable phase shifter, and serves to control the HF source, where the amplitude and phase of reflected and transmitted power can be determined by the AFC. The particle accelerator described provides only one HF coupling-in point, and does not address any problems associated with the coupling in of HF power at multiple points.
Further accelerator structures of a similar type are known from DE 25 19 845 A1, US 2007/0164 237 A1 and DE 1 200 972 B.
The accelerator structures known from the prior art do not permit any scalability in the number of coupling-in points, and do not provide any solution for relieving an HF source by destructive annihilation of backward-travelling wave components from the HF power coupler.
It is the desirable to propose a particle accelerator exhibiting an improved efficiency, so that a given resonator structure can create higher accelerator energies and permits an efficient excitation of the relevant basic frequency for accelerating the particles, where higher modes are attenuated, or an optimum efficiency of the coupling of the electromagnetic energy into the resonator cavity is enabled.
A particle accelerator is proposed in accordance with an aspect of the invention, in particular an electron accelerator that serves to create a bunched particle beam. The particle accelerator comprises an HF source and a directional coupler for splitting an HF power from the HF source of an HF side into at least a first and a second HF power coupler of a cavity side for coupling the HF power into at least one accelerator cavity. It is proposed that a non-reciprocal phase shifter is inserted on the cavity side between the directional coupler and the second HF power coupler, and that an HF load is connected to the directional coupler on the HF side. The non-reciprocal phase shifter is designed such that a reflected HF wave of the second HF power coupler passes through in the direction of the directional coupler with a phase delay such that a destructive interference results between the reflected HF waves of the first and second power couplers in the directional coupler in the direction of the HF source on the HF side.
In other words, a particle accelerator is proposed that comprises at least one accelerator cavity with a plurality of accelerator resonator elements. To supply HF power at two different coupling-in points of the cavity or at two sequential cavity sections, HF power of an HF source is split by means of an HF power coupler into two HF strands. In the first HF strand, HF power is supplied by a first power coupler into a first cavity of the accelerator structure. In the second HF power strand, a non-reciprocal phase shifter is active, by which HF power can be coupled, with phase delay, via a second power coupler into a second HF cavity region of the resonator structure. HF power travelling back in the direction of the HF source is reflected in both the power couplers. The reflected HF wave of the second power coupler is phase-delayed by the non-reciprocal phase shifter such that it is superimposed in the directional coupler on the reflected HF power of the first power coupler such that destructive interference results, so that the HF source is not subjected to reflected HF power. The excess reflected HF energy can be diverted to a connected HF load which also is connected to the directional coupler on the HF side. The result of this is that the HF source works at an ideal efficiency and is not subjected to reflected HF power. It is, accordingly, terminated with the correct impedance, and can guide the entire HF power into the resonator cavity, since no reflected HF power flows backwards. The non-reciprocal phase shifter permits a phase offset for the HF power flowing into the second power coupler in such a way that it can be optimally coupled, with the correct phase, into the second coupling-in region of the resonator cavity. A reflected HF power is delayed in phase in such a way that it is practically extinguished with the reflected HF power of the first power coupler, and that the reflected HF power that remains is diverted into the HF load. This results in an optimum efficiency, so that a high acceleration performance can be achieved even with a simply designed resonator cavity. Higher energies can be created with a more economical and smaller resonator structure.
In an advantageous development of the invention, the directional coupler can be a 4-port directional coupler, in particular a 3 dB directional coupler. In a 3 dB directional coupler, which is also known as an HF power splitter, there is a connection in the main branch between the terminals P1 to P2, and P3 to P4. In addition, an incoming wave that is reflected at port P3 is coupled to the output P4, and in the same way an incoming wave at port P1 is also output to port P3; these coupling branches are therefore represented with crossed arrows in the centre. A directional coupler of this type is also referred to as a forward coupler with four ports. The directional coupler permits reflected power to be transported to the HF load, where the HF source can discharge energy into the accelerator structure with an optimum efficiency.
In an advantageous development of the invention, the non-reciprocal phase shifter can be configured to pass through an adjustably changeable phase delay of the reflected HF wave. Due to the possibility of a changeable phase delay of the non-reciprocal phase shifter, it is possible, for example in the event of a thermal expansion or of a detuning of the resonator cavity, to adjust the phase delay, and to provide a universal building kit for electron accelerators that can be adapted to specific resonator cavities. It is furthermore conceivable that the phase shifter is electronically controllable and that it can set varying phase shifts in the forward and/or reverse branch, for example when an adjusting signal is given. The HF power coupled in via the second power coupler can thus be adjusted, and the energy of the electron beam thereby regulated. The power of the electron beam can also be regulated by the adjustment of the phase shift of the reflected power in both regions. A universally applicable coupling-in network for coupling HF power into a large number of resonator cavities is thus provided, while on the other hand the possibility is also offered of selective control of the coupled-in HF power, and hence of the power of the particle beam.
At least one second non-reciprocal phase shifter can be advantageously provided, inserted on the cavity side between the directional coupler and an HF power coupler, in particular the first power coupler. In this further development, it is proposed that a second non-reciprocal phase shifter can be activated in a further HF branch, in particular in the HF branch of the first power coupler or in an HF branch of a further power coupler. This results in the possibility of reducing the power in yet further regions, as well as of minimising reflected HF power. By cascading several coupling-in branches with several non-reciprocal phase shifters, a high HF power can be supplied into the resonator cavity with an optimised efficiency. This results in far-reaching, possibilities for controlling the HF power, and hence the particle beam.
It is furthermore conceivable to comprise at least a third HF power coupler, which is connected via an at least second directional coupler to the cavity side of the first directional coupler, and which couples in HF power to the accelerator cavity at a further coupling-in point. In this structure, the possibility emerges of coupling in HF power at least at a third or at further points of the resonator structure. Thanks to a modular structure, whereby several coupling-in branches can be formed, in each of which not-reciprocal phase shifters are provided, the reflected power can be minimised, hence improving the efficiency of the HF source, and the coupled-in power can be controlled. This results in the possibility of providing a particle accelerator with a high power spectrum that operates with optimum efficiency. Advantageously, two, four, or a number 2n of coupling-in points are provided, in order to supply the same quantity of HF energy at each coupling-in point. Each directional coupler splits 50% of the HF energy to the two cavity-side output branches, so that 2, 4, 8 or 2n coupling-in points can each be supplied with the same 50%, 25%, 12.5% or 100%/2n HF energy.
In a development of the development discussed above in accordance with the invention, a second HF load can be connected to an HF side of the second directional coupler. As a result of the fact that with a modular structure of at least three or more coupling-in points, a second or more directional couplers are provided, and a further HF load can be connected at least at the second or more directional couplers, reflected HF powers can be absorbed in different HF loads, so that the strain on the overall network of the first HF load is reduced. As a result of this, the possibility emerges, in particular in the case of high-energy applications, of achieving a high level of power and of providing an energy-rich particle beam.
On the basis of the aforementioned further development of a particle accelerator with modular structure having at least three coupling-in points, it can furthermore be advantageous if a further non-reciprocal phase shifter can be inserted between the first directional coupler and the second directional coupler. With a modular structure, therefore, phase shifters can be inserted between the individual directional couplers, so that each phase shifter is designed to delay the phase of a wave reflected in this branch from the several coupling-in points in such a way that it can be superimposed on the respective previous reflected wave with the correct phase. This permits a destructive interference to be achieved in each modular construction stage, so that the entire reflected HF power does not have to be passed back to the first directional coupler, but rather can already be degraded in further modular stages.
A further advantageous exemplary embodiment can furthermore comprise an HF switching element that can disconnect the second power coupler from the directional coupler. The second HF switching element can be designed as an electronic or mechanical switching element, and can switch the HF supply in the branch to the second power coupler on or off, so that the coupled-in HF power can be increased or reduced. This permits a switchable increase or decrease in the HF acceleration energy, in order to permit further control of the energy of the particle beam. It is of course understood that with a modular construction of more than 2 supply points, HF switching elements can be provided in each further HF supply branch.
In a subsidiary aspect, a method is proposed for the operation of a particle accelerator as described above, in which the phase delay of the non-reciprocal phase shifter is adjusted such that a reflected HF wave of the second power coupler is superimposed on a reflected HF wave of the first power coupler in the directional coupler in such a way that a destructive interference of the returning HF waves on the HF side results in the direction of the HF source. In accordance with the invention, a tuning specification is given on how the phase delay of the non-reciprocal phase shifter of the returning wave from the power coupler in the direction of the HF source is to be adjusted in order to achieve a destructive interference with the reflected HF wave of the first power coupler, so that no strain on the HF source results in the directional coupler, and the excess reflected power can be diverted into the HF load. In the case of adjustable non-reciprocal phase shifters in particular, this creates the possibility of being able to adapt a universal HF power electronics twit to any cavity structures in order to ensure an optimum operation of a particle accelerator.
In an advantageous development of the aforementioned method, the delay of the non-reciprocal phase shifter can be controllable. The control, in particular the electronic control, of the phase shifter enables the power of the HF coupling-in to be adjusted over a wide range and the particle beam energy to be made controllable. The adaptation of the HF supply network to any resonator cavities is also enabled.
On the basis of the previous further development of the method the controllable phase delay of the non-reciprocal phase shifter can regulate an HF power input into the accelerator cavity. Two effects are enabled in this way, namely the regulation of the total HF power that can be coupled into the resonator cavity and the elimination of reflected waves in the direction of the HF source, so that an optimum efficiency of the HF side of the particle accelerator can be achieved, and controllability of the particle beam energy is made possible.
The same reference numerals have been used to identify elements that are identical or similar in the figures.